The invention relates to a light-transmissive substrate having a light-transmissive, low-ohmic coating and in particular to a cathode ray tube comprising a display screen having an electroconductive coating.
The invention also relates to a method of manufacturing an electroconductive coating on a substrate.
Electroconductive coatings are used inter alia, as anti-static layers on display screens of display devices, in particular cathode ray tubes (CRTs). Said layers have a sheet resistance of, for example, 106 to 1010 xcexa9/xe2x96xa1 and are thus sufficiently electroconductive to ensure that a high electrostatic voltage present on the outside surface of the display screen is removed within a few seconds. Thus, the user does not experience an unpleasant shock if he touches the screen. Besides, the attraction of atmospheric dust is reduced.
Since it may be hazardous to health, shielding from electromagnetic radiation is becoming ever more important. Devices provided with cathode ray tubes, such as display tubes for TVs and monitor tubes, comprise a number of radiation sources which may be hazardous to the user""s health if he is exposed to said sources for a long period of time. A substantial part of the electromagnetic radiation generated can be screened off with metal in a simple manner via the housing of the cathode ray tube. However, radiation emitted via the display screen may substantially add to the amount of radiation to which the user is exposed.
This problem is solved by applying a well-conducting coating on the surface of the display screen. Said coating must also be sufficiently transparent in the wavelength range from 400 to 700 nm, i.e. the transmission should be at least 60%. A well-known material which can be used for a transparent and well-conducting coating which meets said requirements is indium-doped tin oxide (ITO). Such a layer can be provided by means of vacuum evaporation or sputtering. Said method requires, however, expensive vacuum equipment. ITO layers can also be manufactured by firing spin-coated or sprayed layers of solutions of indium-tin salts. Said firing operation should be carried out at a temperature of at least 300xc2x0 C. This temperature is much too high to be used with a complete display tube which, in order to preclude damage to parts of the display tube, can withstand temperatures of typically 160xc2x0 C.
In German Patent Application DE-A-4229192, a description is given of the manufacture of an anti-static coating for, inter alia, a display screen, said coating being made from poly-3,4-ethylene dioxythiophene and a trialkoxysilane to improve the adhesion. By way of example, a coating is manufactured by providing a desalinated aqueous solution of poly-3,4-ethylene dioxythiophene, polystyrene sulphonic acid and 3-glycidoxypropyl trimethoxysilane on a glass plate, whereafter said glass plate is dried. Said poly-3,4-ethylene dioxythiophene is previously prepared by oxidatively polymerizing the monomer 3,4-ethylene dioxythiophene by means of an Fe(III) salt in water in the presence of polystyrene sulphonic acid to preclude precipitation. The anti-static layer thus obtained has a thickness of 0.6 xcexcm (600 nm) and a sheet resistance of 50 kxcexa9/xe2x96xa1. This sheet resistance is sufficient to bring about an antistatic effect.
A disadvantage of said known layer is that such layers have relatively poor mechanical properties, especially a low scratch resistance, and they are not well suited to be used in anti-reflection coatings because of the relatively low index of refraction of said layer.
It is an object of the invention to provide, inter alia, a substrate, like a display screen of a cathode ray tube, having a coating, said coating providing an effective shield against electromagnetic radiation and exhibiting good optical and mechanical properties. A further object of the invention is to provide a simple method of manufacturing such light-transmissive well-conducting optical coatings, while it is preferably possible, in particular, to carry out said method at relatively low temperatures (typically up to 170xc2x0 C.) at which no damage is caused to parts of a cathode ray tube.
A coated substrate as described in the opening paragraph is characterized in that the coating is provided on a substrate containing SiO2, as a major component the coating comprising a first layer and a second layer adjacent to each other, each of the first and second layers comprising SiO2 as a major component, wherein the first layer comprises a high index material having an intrinsic index of refraction which is higher than the index of refraction of the substrate, and the second layer has an index of refraction which is lower than the index of refraction of the substrate and comprises a conductive polymer, the coating having a sheet resistance below 10 kxcexa9/xe2x96xa1.
In accordance with the above-mentioned requirements, such a layer provides an excellent shield against electromagnetic fields. The conductivity of the coating is primarily due to the conductive polymer. A major component of the substrate, the first and second layer is SiO2 (in this respect it is remarked that, within the framework of the invention SiO2 is to be broadly interpreted and includes glasses comprising SiO2 in whatever form, such as float glass and screen glass used for and in cathode ray tubes). This allows a good adhersion of the layers to each other and to the substrate. The thermal coefficients of expansion of the layers and the substrate are comparable because a major component is the same. As regards layers of pure ITO deposited on glass, this increases the adhersion between the glass and the layer and reduces the occurrence of thermal tensions during or after manufacturing. Coatings of pure conductive polymer lack sufficient scratch resistance. However, the conductive coating in accordance with the invention, comprises a layer of mainly SiO2 and is much more scratch-resistant. In the coating in accordance with the invention, the first layer has an index of refraction which is higher than the index of refraction of the substrate, due to the material having an intrinsic (i.e. as a bulk material) index of refraction which is higher than the index of refraction of the substrate, whereas, due to the conductive polymer, the index of refraction of the second layer is lower than that of the substrate. The combination of a first layer having an index of refraction which is higher than the substrate, and a second layer having an index of refraction which is lower than the substrate allows good anti-reflective properties to be attained. Preferably one of the layers comprises a light-absorbing material. In this manner, it is possible to control the light absorbing properties of the coating. Preferably both the first and the second layer comprise an alkoxy compound. The alkoxy-compound provides a strong mechanical coupling between the coating and the substrate and between the layers. Coatings according to the invention can be applied by means of a wet-coating method, not requiring high temperatures. Metal oxide particles, and in particular ATO, ITO and TiO2 particles are suited for use in the first layer. Particularly useful are particles with a very high index of refraction, i.e. higher than 2.0. The index of refraction of the first layer is thereby substantially increased, which increases the possible use of such layers in anti-reflection coatings. Preferably, each layer is a xc2xcxcex layer, i.e. for each layer the product of the index of refraction of the layer and the thickness of the layer is xc2xcxcex (plus or minus 25%) of the wavelength of visible light (380-780 nm). A suitable thickness range for each of the first and second layers between 50 nm and 150 nm.
ITO and ATO are preferred in those circumstances where a charge build-up of the outer layer of the coating is to be prevented. The at least partly conductive nature of ITO and ATO particles prevents electrical charge from accumulating.
An electroconductive coating in accordance with the invention, optionally with one or more additional layers can also be suitably used as a touch screen coating on a CRT or LCD display screen. By touching a certain part of the touch screen coating on the display screen, a local change in resistance is induced which is translated, via electronic controls, into a localization and a subsequent action, such as opening a menu, turning pages, etc. It is alternatively possible to write on the display screen with a pen, whereafter the writing is identified and processed.
For the additional layer, use may be made of a silicon dioxide layer having a thickness of 50 nm to 250 nm. Using a tetra alkoxysilane, such as TEOS, as the precursor, such a layer can be provided in a simple manner by means of a sol-gel process, followed by curing at a relatively low temperature (about 160xc2x0 C.).
The object of providing a simple method of manufacturing a transmissive electroconductive coating on a substrate (like a display screen of a cathode ray tube) is achieved in an embodiment in that the coating is manufactured by
providing the substrate with a porous layer comprising high index material (such as ITO or ATO) having an index of refraction which is higher than the substrate
subsequently applying a coating solution of a monomer of which the polymer is conductive and an Fe(III) salt on the substrate, whereafter a treatment at an increased temperature is carried out, thereby forming a layer comprising a conductive polymer, (such as poly-3,4-ethylene dioxythiophene) and an Fe(II) salt, after which the double layer is rinsed with an ethanolic solution (e.g. an ethanolic SiO2 precursor, such as e.g. a tetra alkoxysilane like TEOS) which is capable of extracting Fe salts, thereby forming the electroconductive coating. Optionally an organic base can be added to stabilize the system.
The ethanolic solution penetrates the porous layer forming a first layer comprising mainly SiO2, and at the same time extracts the Fe salts from the layer comprising the conductive polymer forming the second layer. This ensures a good adhesion between the first and the second layer and a good adhesion between the first layer and the substrate.
In general, polymers are slightly soluble. In order to obtain a processable polymeric solution, the polymerization reaction is often carried out in the presence of a large quantity of a stabilizing polymer, such as polystyrene sulphonic acid. Said polymer, however, leads to an increase of the sheet resistance. In a preferred embodiment of the method in accordance with the invention, a solution of the monomer instead of a solution of the polymer, is provided on the surface of the display screen. The monomer is subsequently converted to the polymer. The monomer 3,4-ethylene dioxythiophene is converted to the corresponding polymer by means of oxidation with an Fe(III) salt. Fe(III) salts are very suitable because of the redox potential (Ered=0.77 V at room temperature) which is very favorable for this reaction. Fe(III) salts of organic sulphonates are very suitable because of their high solubility in alcohols and low crystallization rate in the liquid layer to be provided. Examples of said salts are Fe(III)-p-toluene sulphonate and Fe(III)-ethylbenzene sulphonate.
Solutions of 3,4-ethylene dioxythiophene monomers and Fe(III) salt, which is necessary for the polymerization reaction, are unstable. When said components are mixed, a polymer soon forms in the solution, as a result of which the pot-life of the coating solution becomes impractically short. Surprisingly, it has been found that the reaction rate of the polymerization reaction is decreased by adding small quantities of a soluble organic base to the coating solution. Dependent on the concentration of the base, the reaction at room temperature can be suppressed completely. In the case of an efficacious base concentration, solutions comprising monomers and the Fe(III) salt can remain stable at room temperature for at least 24 hours: polymerization does not take place. These stable solutions can be used to apply thin layers to the display screen by, for example, spin coating. After heating of the layer, electroconductive poly-3,4-ethylene dioxythiophene is formed. Besides, it has been found that the addition of the organic base has a favorable effect on the conductivity of the polymer and hence on the sheet resistance of the conductive coating. Presumably, the organic base forms a complex with the Fe(III) salt, which results in a reduction of the redox potential at room temperature. This leads to a reduction of the reaction rate, so that a more controlled polymerization at an increased temperature takes place and the specific conductivity increases by approximately a factor of two.
Suitable soluble bases for this method include, for example, imidazole, dicyclohexylamine and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).
Said compounds can readily be dissolved in various alcohols, such as isopropanol and 1-butanol. A solution of said compounds, for example, in 1-butanol is used as the coating solution and has a pot-life of approximately 12 hours. Before the coating solution is used, it is preferably filtered over an 0.5 xcexcm filter.
The coating solution can be provided on the substrate, like a CRT or LCD display screen, by means of customary methods, such as spraying or atomizing. The solution is preferably spin-coated onto the display screen. This results in a smooth, homogeneous and thin layer. During spin coating, the layer provided is dried and subsequently heated to a temperature of typically up to 170xc2x0 C. by means of a furnace, a jet of hot air or an infrared lamp. At a temperature between 100xc2x0 C. and 150xc2x0 C., the polymerization reaction is completed within 2 minutes. The increased temperature initiates the polymerization reaction in which the Fe(III) salt is converted to the corresponding Fe(II) salt. The color of the coating changes from yellow to bluish green. The eventual thickness of the coating depends on the number of revolutions during spin-coating and on the concentration of the dissolved compounds.
The Fe(III) and Fe(II) salts must be removed from the polymerized coating to prevent a dull layer as a result of crystallization. In addition, the Fe(II) salt leads to an increase of the sheet resistance of the coating by a factor of ten. The Fe salts are removed by rinsing the coating with a suitable solvent. In this process, the Fe salts are extracted from the coating. This leads to a strong reduction of the thickness of the coating, increasing the density of the conducting polymer and substantially increasing the conductance of the coating. Surprisingly, rinsing with an ethanolic SiO2 precursor like TEOS results in a mixed Polymer/SiO2 layer having attractive optical (anti-reflective) mechanical (scratch resistance) and electrical (conductance) properties.